An increase in popularity and processing capability of mobile devices, such as laptop computers, cellular phones, Personal Digital Assistants (PDAs), digital cameras, Moving Picture Experts Group (MPEG) layer 3 (MP3) players, Personal Media Players (PMPs), etc., combined with an increase in high-speed wide area networks, has resulted in a large market for small-sized and low-priced image projection systems. Image projection systems can be used independently or installed in mobile devices. Like a mobile device, an image projection system must be small, low-priced, and have high battery efficiency.
Nowadays, a portable image projection system generally uses a light source, an illumination optics, a Spatial Light Modulator (SLM), and a projection optics. The light source and illumination optics uniformly illuminate the SLM, the SLM modulates the intensity of light pixel by pixel, and the projection optics projects and focuses modulated pixels onto an appropriate surface.
Most projection systems use a white light bulb, and the white light is required to be separated into red, green and blue components, which are the three primary colors, for independent modulation. General performance of recent projection systems has reached an eXtended Graphics Array (XGA) resolution (1024×768 pixels) and 24-bit colors (8 bit, i.e., 256 intensity levels per primary color).
FIG. 1 illustrates a conventional image projection system using a white light source 11, a color wheel 12, an illumination optics 13, a two-dimensional (2D) SLM 14, a projection optics 15, and a screen 16.
A projection system uses 3 SLMs for the three primary colors or a single SLM that is illuminated by the three primary colors in sequence as shown in FIG. 1. While a single-SLM system emits only one of the three primary colors at a time, a 3-SLM system simultaneously emits all the three primary colors and thus is brighter than the single-SLM system. Since an SLM is the most expensive component in a projection system, the 3-SLM system is far more expensive than the single-SLM system. In addition, the 3-SLM system requires an additional optics for accurately combining respective red, green and blue images, thus increasing its complexity and cost.
Recently, some projection systems have begun to use a solid-state light source, such as a light-emitting diode (LED) or a laser diode (LD). The solid-state light source emits a single color, and does not need a color separation optics. In addition, the solid-state light source provides electrical efficiency, high color saturation and a wide color gamut.
Most general SLMs are Liquid Crystal Displays (LCDs), Liquid Crystal On Silicon (LCOS) devices, and Digital Mirror Devices (DMDs), all of which are 2D devices having single modulation devices for respective image pixels. In the case of the XGA resolution, the devices have 768432 (1024×768) separate modulation devices. Such SLMs have an active image area of 78.6 mm2 at a pitch of 10 μm.
Most SLMs are fabricated on a semiconductor or a glass wafer. Wafer processing cost is independent of the number of devices, i.e., dies that the wafer includes. Thus, the smaller the surface area of an SLM, the more SLM dies can be fabricated on a wafer. Consequently, more SLM dies can be produced, and the unit price of SLMs can be reduced. However, a die size is substantially limited for reasons relating to cutting, handling and packaging. The smallest reasonable die size is about 1 mm and does not exceed 10 cm. This is because, when a problem occurs in packaging and reliability, an aspect ratio becomes an important characteristic. In addition, a small SLM allows a smaller illumination optics and projection optics to be used more simply and inexpensively.
Conventional 2D SLMs have difficulty matching the core technology for a low-priced small projector. It is difficult to miniaturize the 2D SLMs, and the reduction in pixel size leads to a very complicated and high priced production process. For example, the Texas Instrument Company has reduced the pixel size of a DMD but it is still 16 μm2 to 14 μm2 (). An LCOS pixel is generally smaller than a DMD pixel, but is still at least 8 μm2 (). Thus, a conventional SLM is too large and expensive to be incorporated into a low-priced small image projection system. There is a potentially low-priced method of implementing a smaller image projection system using a one-dimensional (1D) SLM, wherein a single row of modulation devices generate a line image, and rapidly scan the line image forward and backward to generate a 2D image.
Such an SLM may have a remarkably small surface area and is inexpensively produced, thus enabling implementation of a smaller and lower-priced projection system.
FIG. 2 illustrates a projection system using a 1D SLM 23 and a separated red, green and blue (RGB) light source 21 according to conventional art. The projection system illustrated in FIG. 2 has the RGB light source 21, an illumination optics 22, the 1D SLM 23, a frame scanner 24, a projection optics 25, and a screen 26. FIG. 3 illustrates time-divided RGB lights of the projection system shown in FIG. 2. Since the SLM 23 of FIG. 2 performs projection using one row, RGB lights are emitted at different times as illustrated in FIG. 3.
In comparison with the 2D SLM 14 of FIG. 1, the 1D SLM 23 has a much smaller size, but still the aspect ratio must be considered. More specifically, an SLM having a 1D architecture may theoretically use a die corresponding to 1×1024 modulation devices, but has no choice but to use a die corresponding to, for example, 50×1024 modulation devices because an actual die has a limited aspect ratio. Needless to say, even when the die corresponding to 50×1024 modulation devices is used, only 1×1024 modulation devices are formed in the SLM having the 1D architecture. This indicates that the 1D SLM 23 does not efficiently use a die.
In addition, the 1D SLM according to conventional art has a disadvantage in that it still has the same width as a 2D SLM. More specifically, assuming that the width of one modulation device is 10 μm, the 1D SLM 23 having 1024 modulation devices according to conventional art has a length of about 10 mm. Furthermore, when the width of the SLM 23 increases, a distance between the illumination optics 22 and the SLM 23 increases in proportion to the increase in width of the SLM 23, and also the total size of a projection system increases. This prevents the projection system from being attached to a portable device, e.g., a cellular phone.
Furthermore, in the 1D SLM 23 according to conventional art, an interval D1 exists between modulation devices, and thus causes several problems. In most cases, the interval D1 between modulation devices is indispensable for many reasons such as a driver circuit, an area required for interconnection with a driver circuit, prevention of physical contact between the modulation devices, maintenance of insulation between the modulation devices, and so on. However, due to the interval D1 between modulation devices, a dark area may be seen between pixels when the screen 26 is observed in close proximity. Furthermore, the width of the 1D SLM 23 increases in proportion to the interval D1. Assuming that the interval between modulation devices corresponds to a half of a width D2 of the modulation devices, the SLM 23 in which the interval D1 exists between modulation devices has a 50% width excess in comparison with a case in which there is no interval between the modulation devices. This increases the production cost of the SLM and the size of an image projection system.